Greenhouse effect due to CO2 has been pointed out as a cause of global warming in recent years, and a measure against it has become an urgent issue internationally for protecting the global environment. Generation sources of CO2 include all sorts of human activity areas that burn fossil fuel, and demands for the reduction of discharge thereof are further increasing. Accompanying this trend, a method of removing and recovering CO2 in combustion flue gas by bringing combustion flue gas in a boiler into contact with amine CO2 absorbent, and a method of storing recovered CO2 without discharging it to the air have been strenuously researched, with regard to power generation facilities such as a thermal power plant that uses a large amount of fossil fuel.
As a process of eliminating and recovering CO2 in combustion flue gas by using the CO2 absorbent, a process in which combustion flue gas is brought into contact with the CO2 absorbent in an absorber, and a process in which the absorbent having absorbed CO2 is heated in a regenerator to release CO2 and regenerate the absorbent so that the absorbent is circulated and reused in the absorber have been adopted (see, for example, Patent Documents 1 and 2).
As shown in FIG. 4, a conventional CO2 recovering system 1000A has such a configuration that flue gas 1002 containing CO2 exhausted from industrial facilities 1001 such as a boiler is cooled by cooling water 1003 in a cooling column 1004, and the cooled flue gas 1002 containing the CO2 is brought into countercurrent contact with CO2 absorbent containing alkanolamine as a base (amine solution) in an absorber 1006, with CO2 in the flue gas 1002 being absorbed by the CO2 absorbent, thereby eliminating CO2 from the flue gas 1002. The CO2 absorbent having absorbed CO2 (rich solution) 1007 releases CO2 in a regenerator 1008, so that most of CO2 is removed until reaching a bottom part of the regenerator 1008, to regenerate the absorbent as lean solution 1009. The regenerated lean solution 1009 is fed again to the absorber 1006 as the CO2 absorbent (amine solution) and reused.
In FIG. 4, reference letter or numeral 1001a denotes a flue gas duct of the industrial facilities 1001 such as a boiler and a gas turbine, 1001b denotes a chimney having a damper therein, 1010 denotes a blower that supplies flue gas, 1011 denotes purged gas in which CO2 has been removed, 1012 denotes a feed pump for the rich solution 1007 provided on a first feed line L1, 1013 denotes a heat exchanger that heat-exchanges the rich solution 1007 with the lean solution 1009, L3 denotes a CO2 discharge line, 1016 denotes a condenser that condenses water vapor, 1017 denotes a liquid-vapor separator that separates carbon dioxide (CO2) 1018, 1019 denotes a pump for returning the amine solution entrained after CO2 removal to the regenerator 1008, 1020 denotes a pump provided on a second feed line L2 to feed the lean solution 1009, and 1021 denotes a cooling system that cools the lean solution 1009.
The carbon dioxide (CO2) recovered is compressed by a CO2 compressor 1022, thereby acquiring high-pressure CO2 gas 1023 of 10.0 to 15.0 megapascals (G).
The CO2 recovering system can be provided afterwards for recovering CO2 from an existing flue gas source, or can be provided at the same time of newly installing a flue gas source.
A reboiler for evaporating a part of the extracted amine solution is installed at the bottom of the regenerator 1008. The evaporated equilibrium vapor becomes stripping vapor for giving energy for amine-CO2 dissociation.
Because the amine solution for absorbing CO2 is not tolerant to heat and is decomposed at a high temperature, although a high temperature is desired in view of stripping performance. Therefore, the regenerator 1008 operated at a temperature as low as possible, taking thermal decomposition into consideration. A reboiler heat source is also limited up to 150° C.
In view of avoiding decomposition, it is preferred that a contact time of the amine solution with the heat source is as short as possible.
As a type of the reboiler, a horizontal thermo-siphon reboiler and a kettle-type reboiler have been conventionally used.
In FIG. 4, an example of using a horizontal thermo-siphon reboiler 1030 is shown.
The horizontal thermo-siphon reboiler 1030 has a heat-transfer tube 1032 to which low-pressure steam 1031 is supplied. The heat-transfer tube 1032 heats CO2 absorbent (amine solution) 1033 extracted from the regenerator 1008, separates carbon dioxide therein, and returns it to inside of the regenerator 1008 as gas-liquid two phase flow 1034. Reference numeral 1035 denotes condensed water.
The horizontal thermo-siphon reboiler 1030 is normally used as a reboiler of a distillation column. However, it has a problem that evaporated vapor and liquid become mixed phase gas-liquid two phase flow 1034 to pass through the heat exchanger and an outlet piping, thereby increasing flow resistance. Therefore, a boiling point rise of 2 to 5° C. occurs in the heat exchanger.
As a result, thermal decomposition of the amine solution therein becomes a problem.
To suppress the boiling point rise, there is a case that a kettle-type reboiler 1040 is installed instead of the horizontal thermo-siphon reboiler 1030, as shown in a CO2 recovering system 1000B in FIG. 6.
The kettle-type reboiler 1040 heats the amine solution 1033 by using a heat-transfer tube 1041, to which low-pressure steam 1031 is supplied, separates carbon dioxide therein, extracts it as vapor 1042 containing carbon dioxide from the top of the kettle-type reboiler 1040, and returns it to the inside of the regenerator 1008.
In the kettle-type reboiler 1040, the separated lean solution 1009 is separated by a gate 1043, and returned to a liquid residence section 1045 of amine solution at the bottom of the regenerator 1008.
In the kettle-type reboiler 1040, the evaporated vapor 1042 and the lean solution 1009 are separated in the heat exchanger and does not form vapor-liquid mixed phase at the outlet piping, and thus most of the flow resistance is only in a heat exchanger tube bundle. The boiling point rise for this is approximately from 0.2 to 1° C. However, a very large shell diameter is required for vapor-liquid separation and a residence time thereof becomes long.
A chemical reaction rate becomes twice when the temperature increases by 10° C., and thus it is preferred that the residence time in the kettle-type reboiler 1040 is as short as possible.
It is also preferred that the pressure is as low as possible, because the regenerator 1008 is operated at a low temperature for suppressing decomposition. However, because the recovered CO2 is compressed in multi stages, by increasing the pressure on a suction side, compression power of the CO2 compressor 1022 can be reduced. Therefore, an efficient regeneration system has been desired.    Patent Document 1: Japanese Patent Application Laid-open No. H06-91134    Patent Document 2: Japanese Patent No. 3716195